Postfix attempts to be fast, easy to administer, and secure.
The outside has a definite Sendmail-ish flavor, but the inside is
completely different.

Newsflash

Postfix does not call gethostbyname() since 2005. There is no
Postfix code that invokes this function unless Postfix is specifically
built for operating systems from more than 10 years ago (this
requires the compile-time option -DNO_IPV6).

About this website

This website has information about the Postfix source code
distribution. Built from source code, Postfix can run on UNIX-like
systems including AIX, BSD, HP-UX, Linux, MacOS X, Solaris, and
more.

Postfix is also distributed as ready-to-run code by operating
system vendors, appliance vendors, and other providers. Their
versions may have small differences with the software that is
described on this website.